Highly refractory body



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[VOR E. CAMPBELL BRUCE W GOMSER CARROLL E POWELL INVENTORS Jan. 12, 1954 l. E. CAMPBEL L ETAL HIGHLY REFRACTORY BODY Filed March 18, 1950 2 Sheets-Sheet 2 HFE IN HOUR 8 :o

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Patented Jan. 12, 1954 HIGHLY REFRACTORY BODY Ivor E. Campbell, Gahanna, and Bruce W. Gonser and Carroll F. Powell, Columbus, Ohio, assignors, by mesne assignments, to Fansteel Metallurgical Corporation, North Chicago, Ill., a corporation of New York Application March 18, 1950, Serial No. 150,398

1 Claim.

This invention relates to metal bodies resistant to oxidation at high temperatures and more particularly to such bodies formed of molybdenum which is provided with a coating or skin to render the base metal resistant to oxidation at high temperatures.

The refractory metal molybdenum has highly desirable properties and characteristics for many purposes. For example, it retains its mechanical properties at elevated temperatures and is very desirable for use in forming electrical furnace heating elements. However, in order to prevent oxidation of the metal at elevated temperatures it is necessary to exclude oxygen and a continuous flow of hydrogen is maintained over the heated resistance elements. Molybdenum, of course, is desirable for many other applications, the foregoing use being merely the best known.

The principal purpose of the present invention is to provide molybdenum bodies having a coating or skin which is resistant to oxidation at high temperatures and which thereby protects the base or core from oxidation at elevated temperatures.

Other objects and advantages of this invention will become apparent from the description which follows.

The present invention contemplates bodies of the refractory metal molybdenum, normally oxidizable at high temperatures, having a coating or skin of molybdenum-silicon alloys or intermetallic compounds. 7

Methods are known whereby silicon may be deposited as an adherent coating on molybdenum. One of the methods includes passing a mixture of hydrogen and vapors of a silicon halide over the heated refractory metal. The protection afforded by the silicon coating or film is limited by the melting point of silicon and by its tendency to alloy with the base material. For many purposes the melting point is sufficiently high to afford some degree of protection for the refractory metal base or core.

The molybdenum-silicon skin or coating of our invention affords protection against oxidation at appreciably higher temperatures. This skin or coating may be formed by any desired method.

In the production of the molybdenum bodies having the alloy skin or coating, silicon may be applied to or deposited on the metal and the coated metal maintained in a protective atmosphere at a temperature of the order of the melting point of silicon. During this treatment the molybdenum and silicon alloy or react with each other to form an alloy or intermetallic compound. The alloy coating may also be formed on the molybdenum base by maintaining the metal at higher temperatures during the deposition of silicon. If the metal base is maintained at temperatures above or in the neighborhood of the melting point of silicon, the alloy or intermetallic compound is formed directly.

We have discovered that the alloy coatings are superior to the silicon coating not only in their ability to protect the metals at higher temperatures but also in their greater adherence to the metal base, being more or less integral with the metal base. The precise chemical compositions of the alloy coatings or skins appear to have a direct bearing upon the oxidation resistance of the bodies. The silicon content of the coatings or skins may vary in the molecular ratio of silicon to refractory metal of from about 1:1 to about 3:1, which corresponds to alloys or intermetallic compounds containing from about 22.5% to about 47% silicon. An alloy or compound cor responding to a molecular ratio of silicon to molybdenum of about 2:1, or containing about 37% silicon, appears to exhibit optimum protection against oxidation at elevated temperatures' However, it is to be understood that coatings or skins outside this composition range also afford some protection for the metal base or core. The alloy coatings or skins in the above composition range appear to be somewhat more ductile than substantially pure silicon coatings, particularly at elevated temperatures.

The protection of molybdenum against oxidation offered by the alloy coatings or skins may be illustrated by the following example:

Molybdenum wire having a diameter of 0.020 inch was coated by the deposition of silicon from a mixture of hydrogen and silicon tetrachloride. The wire was first heated in a, chamber having a hydrogen atmosphere and, after it reached an elevated temperature as noted below, silicon tetrachloride vapors and hydrogenwere introduced into the chamber. The temperature at which various samples were maintained during Wire temperature 'Ighickness o coating, In hydro- Q F inch gen, C.

pors, G.

The resistance of the coated wire to oxidation at high temperatures was measured by mounting between suitable electrodes a 12/2 inch length of each filament and passing an electric current through the wire. The current was maintained at such a value so as to maintain the filament temperature, as measured by an optical pyrom'eter, at a temperature of about 1500 C. (273? F.). The life of the filament when operating at this temperature in the open air was measured in seconds. As a control, an uncoated molybdenum filament of the same size and length was likewise heated in the air. The following data illustrates the high temperature oxidation resistance of these types of coatings:

Life-At 1500 C. in air Wire temperature during costing Time to an, In hydm SIjDQFz-I: seconds gen C. O

' ipors, C.

t Uncoated 16 ,000 15 1,420 1,280 360 1, 600 1, 410 1,820 1, 620 x 1, 460 2, 250 1,580 1, 650 4, 000 1, 750 1, 650 4, 740

These data are also shown in Fig. l of the accompanying drawings. It will be noted that there is substantially no difference between the life at 1500 C. of an uncoated wire and a coated wire where the plating has been such as to produce a coating in which the predominating constituent is silicon. Since the filament was maintained in air at a temperature considerably above the melting point of silicon during the test, some of the silicon is lost by vaporization and some of the silicon difluses into or interacts with the molybdenum non-uniformly producing an irregular and discontinuous skin or coating. The wire, therefore, is not coated uniformly with the alloy skin and when subjected to this high temperature fails rapidly at the uncoated areas. In those instances where the silicon was de posited while the molybdenum was maintained at a temperature of the order of or exceeding the melting point of silicon, the silicon and molybdenum react immediately and the predominating constituent of the coating is the alloy or intermetallic compound. This type of coating or skin is appreciably more resistant at the temperatures ing periods.

employed in the tests as clearly shown by the life curve, Fig. 2.

It is apparent that the life of the treated molybdenum will be greater at lower temperatures. This may be illustrated by the following example:

Molybdenum wire samples having a diameter of 0.080 inch were provided with a skin of approximately 0.001 inch in thickness. These samples were provided with the skin or coating by maintaining the samples at 1600 C. in an atmosphere of hydrogen and silicon tetrachloride vapors for about seconds. The treated filaments were'then heated in air by passing an electric current through the filaments. The life of the filaments operating at the designated temperatures was measured and were as follows:

Temperature, 0. Life 977 hours (without failure). 128.1 hours.

62.2 hours.

27.1 hours.

The oxidation resistance of molybdenum may be further increased by applying the coating or skin in successive layers with intermediate soak- During the intermediate soaking periods the temperature is maintained sufficiently high to insure a complete formation of the alloy or intermetallic compound. For example. a molybdenum wire which had been coated by four applications had a life at 1700 C. (3092 F.) in air of 159.6 hours. In the preparation of this filament, the wire was subjected to four coating periods, each coating period being followed by a heat treatment consisting of maintaining the coated wire at about 1750 C. in hydrogen for about five minutes.

Although for many purposes, tungsten, tantalum and columbium have been considered as substantially equivalent to molybdenum, we have discovered that this equivalency does not obtain for the purposes of the present invention. These other common refractory metals, although they may be provided with similar coatings or skins of silicon alloys, do not have oxidation resistant characteristics which are comparable to the mo lybdenum products of our invention. This difference in characteristics may be illustrated by reference to certain experimental investigations covering the study of the four refractory metals.

Samples of molybdenum, tungsten, tantalum and columbium in the form of wire having a diameter of 0.080 inch were provided with a skin of approximately 0.001 inch in thickness by depositing the silicon and forming the alloys or in termetallic compounds of the respective metal and silicon as described hereinbefore. A number of such samples of each of the treated or coated metal wires were heated to about 1000 C. in air by passing an electric current through the wires. The average life of these samples at this temperature was as follows:

Although this invention has been illustrated by a description of the formation of the alloy skins or coatings by deposition of the silicon on the molybdenum base from a vaporized silicon compound and hydrogen, other methods may be employed. For example, the silicon may be deposited by a spraying operation or'other suitable method so as to provide a silicon coating. The coated molybdenum body is then subjected to a heat treatment, as by heating in a hydrogen atmosphere or under other suitable conditions, in order to cause or permit a difiusion of and interaction between the silicon and the molybdenum thereby forming the highly protective alloy or intermetallic compound coating or skin.

It is to be understood that the foregoing description is merely illustrative of our invention and that the specific examples set forth are not intended as limitations. Although the specific examples have described treated molybdenum wire, bars, strips and other bodies have been similarly protected against oxidation at elevated temperatures. It is apparent that the description of the molybdenum wires affords a ready illustration of the efiectiveness of the coatings or skins and of the comparison of the bodies of this invention as compared to other common refractory metals.

It is to be understood that the expression in the claim, an exterior layer composed predominantly of an intermetallic compound of molybdenum and silicon consisting of about 37% silicon and the balance molybdenum, does not mean that the exterior layer consists entirely of the intermetallic compound of silicon and molybdenum having the approximate percentage named, namely, MoSiz, but that the exterior layer consists largely or essentially'of that compound and may have associated with it the compound of molybdenum and silicon consisting of about 22.5% silicon and the balance molybdenum, namely MoSi, and/or silicon in an amount in excess of that required to form the compound MOSiz, up to a total silicon content of about 47%.

We claim:

As an article of manufacture, a refractory metal body comprising a molybdenum base having an exterior layer composed predominantly of an intermetallic compound of molybdenum and silicon consisting of about 37% silicon and the balance molybdenum, the body being characterized by having a high resistance to oxidation at temperatures above about 1000 C.

IVOR E. CAMPBELL.

BRUCE W. GONSER. CARROLL F. POWELL.

References Cited in the file of this patent UNITED STATES PATENTS Number 

